.\"   $OpenBSD: shmget.2,v 1.2 1996/10/08 01:20:16 michaels Exp $
.\"	$NetBSD: shmget.2,v 1.1 1995/10/16 23:49:32 jtc Exp $	
.\"
.\" Copyright (c) 1995 Frank van der Linden
.\" All rights reserved.
.\"
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.\" modification, are permitted provided that the following conditions
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.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
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.\" 3. All advertising materials mentioning features or use of this software
.\"    must display the following acknowledgement:
.\"      This product includes software developed for the NetBSD Project
.\"      by Frank van der Linden
.\" 4. The name of the author may not be used to endorse or promote products
.\"    derived from this software without specific prior written permission
.\"
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.\"/
.Dd August 17, 1995
.Dt SHMGET 2
.Os
.Sh NAME
.Nm shmget
.Nd get shared memory area identifier
.Sh SYNOPSIS
.Fd #include <sys/shm.h>
.Ft int
.Fo shmget
.Fa "key_t key"
.Fa "size_t size"
.Fa "int shmflg"
.Fc
.Sh DESCRIPTION
.Fn shmget
returns the shared memory identifier associated with the key
.Fa key .
.Pp
A shared memory segment is created if either
.Fa key
is equal to IPC_PRIVATE, or
.Fa key
does not have a shared memory segment identifier associated with it, and the IPC_CREAT
bit is set in
.Fa shmflg.
.Pp
If a new shared memory segment is created, the data structure associated with it (the
.Va shmid_ds
structure, see
.Xr shmctl 2 )
is initialized as follows:
.Bl -bullet
.It 
.Va shm_perm.cuid
and
.Va shm_perm.uid
are set to the effective uid of the calling process.
.It
.Va shm_perm.gid
and
.Va shm_perm.cgid
are set to the effective gid of the calling process.
.It
.Va shm_perm.mode
is set to the lower 9 bits of
.Fa shmflg .
.It
.Va shm_lpid ,
.Va shm_nattch ,
.Va shm_atime ,
and 
.Va shm_dtime
are set to 0
.It
.Va shm_ctime
is set to the current time.
.It
.Va shm_segsz
is set to the value of
.Fa size .
.It
The
.Xr ftok 3
function may be used to generate a key from a pathname.
.El
.Sh RETURN VALUES
Upon successful completion a positive shared memory segment identifier is returned.
Otherwise, -1 is returned and the global variable
.Va errno
is set to indicate the error.
.Sh ERRORS
The
.Fn shmget
system call will fail if:
.Bl -tag -width Er
.\" ===========
.It Bq Er EACCES
A shared memory segment is already associated with
.Fa key
and the caller has no permission to access it.
.\" ===========
.It Bq Er EEXIST
Both IPC_CREAT and IPC_EXCL are set in
.Fa shmflg ,
and a shared memory segment is already associated with
.Fa key .
.\" ===========
.It Bq Er EINVAL
No shared memory segment is to be created,
and a shared memory segment exists for
.Fa key ,
but the size of the segment associated with it
is less than
.Fa size ,
which is non-zero.
.\" ===========
.It Bq Er ENOENT
IPC_CREAT was not set in
.Fa shmflg
and no shared memory segment associated with
.Fa key
was found.
.\" ===========
.It Bq Er ENOMEM
There is not enough memory left to created a shared memory segment of the
requested size.
.\" ===========
.It Bq Er ENOSPC
A new shared memory identifier could not be created because the system limit
for the number of shared memory identifiers has been reached.
.El
.Sh LEGACY SYNOPSIS
.Fd #include <sys/types.h>
.Fd #include <sys/ipc.h>
.Fd #include <sys/shm.h>
.Pp
.Ft int
.br
.Fo shmget
.Fa "key_t key"
.Fa "int size"
.Fa "int shmflg"
.Fc ;
.Pp
All of these include files are necessary.
The type of
.Fa size
has changed.
.Sh SEE ALSO
.Xr ftok 3 ,
.Xr shmat 2 ,
.Xr shmctl 2 ,
.Xr shmdt 2 ,
.Xr compat 5
